U.S. patent application number 11/382811 was filed with the patent office on 2009-08-06 for core suction technique for the fabrication of optical fiber preforms.
Invention is credited to Nitin Kumar Goel, Daniel Kominsky, Steven H. Morgan, Rogers H. Stolen.
Application Number | 20090193851 11/382811 |
Document ID | / |
Family ID | 37431907 |
Filed Date | 2009-08-06 |
United States Patent
Application |
20090193851 |
Kind Code |
A1 |
Goel; Nitin Kumar ; et
al. |
August 6, 2009 |
CORE SUCTION TECHNIQUE FOR THE FABRICATION OF OPTICAL FIBER
PREFORMS
Abstract
Optical fiber preforms which can be drawn into optical fibers of
desired dimensions are fabricated by applying a vacuum to a
cladding tube and drawing molten glass from a crucible into a bore
of the cladding tube while a portion of the cladding tube is within
a furnace preferably through a small hole in the top of the
furnace. The method and apparatus are particularly applicable to
highly non-linear fiber (HNLF) glasses and highly doped or rare
earth glasses since materials therein are generally expensive and
only a small quantity of molten glass is required but can be
applied to virtually any optical fiber construction where the core
glass has a lower melting or softening point than that of the
cladding tube. Sources of contamination, breakage and other preform
defects are substantially avoided and toxic substances, if present
are readily confined.
Inventors: |
Goel; Nitin Kumar; (Uttar
Pradesh, IN) ; Stolen; Rogers H.; (Gallatin, TN)
; Morgan; Steven H.; (Brentwood, TN) ; Kominsky;
Daniel; (Christiansburg, VA) |
Correspondence
Address: |
WHITHAM, CURTIS & CHRISTOFFERSON & COOK, P.C.
11491 SUNSET HILLS ROAD, SUITE 340
RESTON
VA
20190
US
|
Family ID: |
37431907 |
Appl. No.: |
11/382811 |
Filed: |
May 11, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60680045 |
May 12, 2005 |
|
|
|
Current U.S.
Class: |
65/404 ;
65/533 |
Current CPC
Class: |
C03B 37/01268
20130101 |
Class at
Publication: |
65/404 ;
65/533 |
International
Class: |
C03B 37/027 20060101
C03B037/027; C03B 37/03 20060101 C03B037/03 |
Goverment Interests
STATEMENT OF GOVERNMENT INTEREST
[0002] The development of this invention was funded by the National
Science Foundation, grant number ECS-0123484 and the National
Institute of Aerospace, grant number VT-03-1. Accordingly, the
United States Government has certain interests in this invention.
Claims
1. A method of fabricating an optical fiber preform, said method
comprising steps of melting or softening a glass material in a
crucible enclosed in a furnace, introducing one end of a cladding
tube into said furnace through an aperture in said furnace and
below a surface of said glass material, and applying suction to an
opposite end of said cladding tube to draw said glass material into
said cladding tube.
2. A method as recited in claim 1, further including a step of
preheating said cladding tube.
3. A method as recited in claim 1, further including a step of
depositing at least one layer of material within a bore of said
cladding tube.
4. A method as recited in claim 3, wherein said at least one layer
of material is a barrier layer.
5. A method as recited in claim 1, wherein said glass material is a
highly non-linear fiber (HNLF) glass
6. A method as recited in claim 1, wherein said glass material is a
highly doped glass.
7. A method as recited in claim 1 wherein said glass material is a
rare earth glass.
8. A method as recited in claim 1, wherein said cladding tube is
formed of silica.
9. A method of fabricating an optical fiber, said method comprising
steps of melting or softening a glass material in a crucible
enclosed in a furnace, introducing one end of a cladding tube into
said furnace through an aperture in said furnace and below a
surface of said glass material, applying suction to an opposite end
of said cladding tube to draw said glass material into said
cladding tube to form a preform, and drawing said preform into an
optical fiber of desired dimensions.
10. A method as recited in claim 9, further including a step of
preheating said cladding tube.
11. A method as recited in claim 9, further including a step of
depositing at least one layer of material within a bore of said
cladding tube.
12. A method as recited in claim 11, wherein said at least one
layer of material is a barrier layer.
13. A method as recited in claim 9, wherein said glass material is
a highly non-linear fiber (HNLF) glass
14. A method as recited in claim 9, wherein said glass material is
a highly doped glass.
15. A method as recited in claim 9 wherein said glass material is a
rare earth glass.
16. A method as recited in claim 9, wherein said cladding tube is
formed of silica.
17. An apparatus comprising, in combination, a furnace including a
heat source and a crucible containing molten glass, means for
introducing a portion of a cladding tube into said furnace such
that a first end of said cladding tube reaches a surface of said
molten glass in said crucible, and vacuum pump means for applying
vacuum to a second end of said cladding tube to draw said molten
glass into said cladding tube by suction.
18. An apparatus as recited in claim 17, wherein said furnace is a
muffle furnace.
19. An apparatus as recited in claim 17, wherein said furnace
further includes means for directing a desired gas across a surface
of said cladding tube.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of priority of U.S.
Provisional Patent Application Ser. No. 60/680,045, filed May 12,
2005, which is hereby incorporated by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention generally relates to optical fibers
and, more particularly, to preforms for the fabrication of optical
fibers including highly non-linear non-conventional glasses as a
core material thereof.
[0005] 2. Description of the Prior Art
[0006] Fiber optics are becoming of very widespread use for
communication links due to their degree of noise immunity and
potential bandwidth. Numerous designs of low loss optical fibers
are known and commercially available in long lengths and their
performance is well-documented and subject to industry standards.
Fiber optics also have many applications such as in light
amplifiers where nonlinear effects are of primary importance and
require non-standard fibers with very high non-linearity in short
lengths. Such fibers with very high non-linearity glasses in the
core are known as highly non-linear fibers (HNLFs). Additionally,
optical fibers are often made in short lengths of highly doped
glasses which are also drawn from preforms. An example of such
glasses is MM2 glass from Kigre, Inc. which is highly doped with
erbium but not a non-linear glass.
[0007] Attempts to fabricate HNLFs fall into three general
categories which can be classified and are generally referred to as
Modified Chemical Vapor Deposition MCVD techniques, crucible
methods and rod-in-tube, respectively. MCVD techniques which
involve a layer-by-layer deposition of material of a core (which
can form a highly non-linear optical region) inside a cladding
tube, offer the greatest potential for high purity and tight
confinement but the processes are very slow and somewhat
complicated, particularly for multi-component glasses in the core.
(Cladding is a layer of material surrounding the core generally to
increase efficiency of light transmission and reduce light loss.
Such a layer surrounding the core may or may not function as
cladding but the term "cladding" will be used herein as a
collective reference to materials surrounding an optical fiber
core, regardless of its actual function.) Crucible and rod-in-tube
approaches usually involve processing of bulk glass samples or
glass powders and using the cladding tube essentially as a crucible
for the core glass where softening or melting of the core glass
occurs principally during drawing of the optical fiber from the
preform. Core glasses can be processed separately and then combined
with cladding material using all of the above-mentioned techniques.
Also, since HNLFs and highly doped glasses are non-standard, it may
be desirable to include any of a wide variety of materials in the
core glass, some of which may be highly toxic. None of the above
known techniques of fabricating optical fiber preforms lend
themselves particularly well to confinement of substances and
vapors which may be toxic without the addition of particular
structures for that purpose or other significant complication of
the respective processes.
[0008] The most significant difficulty with these techniques
derives from the desired small core diameter of the final fiber
drawn from the preform. Non-linear glasses are usually associated
with very high refractive indices as compared to silica. High
refractive indices lead to a need for a very small core diameter
between 0.5 and 1.0 microns to maintain the fiber single mode. A
small core diameter increases the non-linearity by reducing
effective area. Thus, most such HNLFs are fabricated by drawing
fibers to the final desired size from preforms which can be
substantially larger in cross-sectional dimensions. Even so, a 0.5
micron core in a 125 micron fiber requires that the inner diameter
of a 1 cm cladding tube be only 40 microns. Thus, the core glass
fiber would have to be drawn to a diameter of less than 40 microns
prior to being inserted into a cladding tube (of a type which
allows splicing to other fibers) before the assembly is drawn to
final desired dimensions thus complicating and introducing
additional failure modes into the assembly and drawing processes to
produce the preform and then the optical fiber.
[0009] Further, additional problems have been encountered in known
assembly and fabrication techniques for HNLFs. For example, trapped
air entrained due to the necessary initial clearance between
cladding and core may form bubbles within the cladding and it is
anticipated that such bubbles will become more of a problem as the
core and cladding bore become smaller. There is also a limit to the
size from which a fiber having the desired final dimensions can be
drawn due to the large thermal expansion coefficient (CTE) of high
non-linearity glasses. These assembly and drawing techniques can
also be sources and opportunities for contamination of the fiber at
several stages of these processes.
SUMMARY OF THE INVENTION
[0010] It is therefore an object of the present invention to
provide a technique of forming a preform for a highly non-linear
optical fiber (HNLF) in which core material may be made to
completely fill an inner bore of cladding material or the like and
which can be spliced to other optical fibers.
[0011] It is another object of the present invention to provide an
alternative, rapid, repeatable, robust and economical technique of
producing optical fiber preforms to known techniques of fabricating
optical fiber preforms, particularly for HNLFs.
[0012] It is a further object of the invention to provide an
arrangement for fabrication of fiber optic preforms which allows
safe fabrication even if toxic substances are to be included in the
glass core.
[0013] It is yet another object of the invention to provide a
technique of forming preforms, particularly preforms with very
small diameter cores, and which will decrease the incidence of
breakage and defects.
[0014] In order to accomplish these and other objects of the
invention, a method is provided for fabricating optical fiber
preform or an optical fiber comprising steps of melting or
softening a glass material in a crucible enclosed in a furnace,
introducing one end of a cladding tube into said furnace through an
aperture in said furnace and below a surface of said glass
material, and applying suction to an opposite end of said cladding
tube to draw said glass material into said cladding tube.
[0015] In accordance with another aspect of the invention, an
apparatus is provided comprising, in combination, a furnace
including a heat source and a crucible containing molten glass, an
arrangement such as an aperture for introducing a portion of a
cladding tube into said furnace such that a first end of said
cladding tube reaches a surface of said molten glass in said
crucible, and a vacuum pump arrangement for applying vacuum to a
second end of said cladding tube to draw said molten glass into
said cladding tube by suction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The foregoing and other objects, aspects and advantages will
be better understood from the following detailed description of a
preferred embodiment of the invention with reference to the
drawings, in which:
[0017] FIG. 1 is a schematic depiction of apparatus suitable for
practice of the invention and useful for explanation of the
principles and methodology of the invention,
[0018] FIG. 2 is a photomicrograph of a cross-section of an optical
fiber drawn from a preform fabricated using the apparatus and
technique depicted in FIG. 1, and
[0019] FIG. 3 depicts the output mode pattern of the optical fiber
of FIG. 2 having a red He--Ne light input.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
[0020] Referring now to the drawings, and more particularly to FIG.
1, there is shown a schematic depiction of basic apparatus for
practice of the invention by which the principles and methodology
of the invention sufficient to its successful practice may be
readily conveyed to those skilled in the art. The basic apparatus
sufficient to practice the invention is extremely simple and
suitable structures from which the apparatus may be assembled are
commercially available. Basically, the apparatus comprises a muffle
furnace 10 or other type of furnace preferably dimensioned to
accommodate a principal portion, if not all, of a preform of the
desired length which is generally equipped with a thermometer hole
20. (In general, the preform that can be fabricated will be
substantially limited to the portion of the cladding tube which can
be heated in the furnace since the core glass will generally
solidify quickly upon reaching an unheated portion of the cladding
tube. Common sizes of thermometer holes provided in commercially
available muffle furnaces are normally adequate to accommodate
cladding tubes of sizes generally used. If a thermometer hole is
not provided or is of a size other than to closely fit around the
outer diameter of a cladding tube, a suitable modification can be
easily provided in order to do so. A crucible 30 of alumina or
other refractory material capable of withstanding a temperature
somewhat in excess of the melting point of the intended core glass
or simply a conventional crucible suitable for glass processing is
provided within the furnace 10, along with a heat source 40, the
particulars of which are not important to the successful practice
of the invention. A vacuum pump 50 of any known type but preferably
capable of drawing a vacuum at least sufficient for drawing molten
glass into a cladding tube having small bore dimensions comparable
to those noted above or smaller to fabricate an optical fiber
preform within a sufficiently short time to suitably limit any
significant diffusion or non-uniformity over the length of the
preform in accordance with the invention which can then be drawn to
the desired final dimensions in a manner well-understood in the
art. (The required vacuum will vary with viscosity of the glass
which will, in turn, vary with temperature and thus the required
vacuum levels may be limited to practical levels by suitable choice
of glass temperature.)
[0021] It should be appreciated that the technique of drawing core
glass into the bore of a cladding tube using suction is possible
due to the fact that the melting or softening temperatures of
highly non-linear core glasses and highly doped glasses are much
less than that of the cladding tube which is generally of silica
but lower melting or softening temperature glasses such as
boro-silicate glass can also be used for lower softening or melting
temperature core glasses. By the same token, the invention is
completely applicable to fabrication of preforms for optical fibers
of conventional composition as long as a suitable differential of
melting/softening temperatures between the core glass material and
the cladding material can be provided. Therefore, the bore of the
cladding tube can effectively be used in the manner of a mold for
the core glass which is drawn into it, thus eliminating voids and
ambient gas entrainment and core rod breakage which have been
observed when a solid core rod is inserted into the cladding tube.
Furnace 10, when in use, has only one opening for the insertion of
the cladding tube (and vacuum connection and thus is able to
confine and toxic gasses which may be present or evolved from the
molten glass. Such gasses which are drawn into vacuum pump 50 may
be suitably confined or exhausted. Further, since the core glass is
drawn into the cladding tube in a molten state, it is believed that
incidence of breakage due to differences of the coefficient of
thermal expansion between the cladding and core during drawing of
the preform into an optical fiber at an elevated temperature is
reduced.
[0022] To fabricate a preform using apparatus of the general type
and having the features illustrated in FIG. 1, the materials
required for core glass of the desired composition is charged into
the crucible 30 and melted by application of heat from heat source
40. It is a meritorious effect of the present invention that only a
very small amount of core glass material need be provided and
melted since novel and non-standard glasses to which the invention
is particularly applicable may contain materials which are very
expensive. When the glass in crucible 30 has reached the desired
temperature to provide a suitable viscosity (but below the melting
point of the cladding tube) in view of the bore of the cladding
tube and vacuum level so that the bore can be filled in a suitably
short time, the end of the cladding tube 60 is inserted into the
muffle furnace 10 through the thermometer hole 20 and into the
glass in the crucible 30 and vacuum is applied to the other end of
the cladding tube 60 by vacuum pump 50 to draw the molten core
glass into the cladding tube.
[0023] It will be recognized by those skilled in the art that the
above process involves numerous interrelated parameters and many
possibilities for process variation; each of which may be chosen
within a relatively wide range consistent with successful practice
of the invention. For example, the vacuum can be applied either
before or after the insertion into the muffle furnace and crucible.
The choice may involve such factors as the desired temperature of
the cladding tube and its specific heat which should be such that
the molten core glass does not significantly solidify due to
temperature drop through heat transfer to the cladding tube during
the suction process and drawing the high temperature ambient
atmosphere within the furnace through the cladding tube prior to
suctioning of the glass may allow the cladding tube to reach the
desired temperature more quickly.
[0024] In general, in this regard, it is preferable that the
dimensions of the muffle furnace 10 be such that substantially all
of the cladding tube can be received therein with no more
extension, if any, outside the muffle furnace than is necessary for
connection to the vacuum pump 50. For longer preforms, a so-called
horizontal tube furnace can be oriented vertically to accommodate
the cladding tube vertically therein and used in place of a muffle
furnace. Elongated furnaces such as those conventionally used for
fiber drawing operations can also be used as well as furnaces of
custom dimensions to accommodate preforms of whatever dimensions
may be desired and can include such features as silica liners and
structures 70 such as for passing inert gases or other controlled
atmospheres over the cladding to further reduce the potential for
contamination can be provided as schematically and collectively
illustrated in FIG. 1. (Incidentally, in practice, some difficulty
was initially encountered during insertion of the cladding tube
into the furnace causing displacement or overturning of the
crucible. If a structure such as is schematically depicted at 70 of
FIG. 1 is dimensioned to fit closely around the crucible, it can
also serve to guide the cladding tube to the crucible as well as
supporting the crucible; thus eliminating this early problem.)
Since these other types of furnace differ from a muffle furnace
principally by dimensions, the illustration of furnace 10 should be
considered as a collective illustration of any and all types of
furnaces capable of accommodating regardless of nomenclature. Thus,
the furnace of whatever type is used can provide pre-heating of all
or most of the cladding tube to a temperature which will prevent
the core glass from solidifying during the suction of the core
glass. In this regard, it may be preferable to insert the cladding
tube end into the furnace to a point which does not initially reach
the molten core glass such that drawing ambient furnace gases
through the cladding tube expedites the preheating process, as
alluded to above. On the other hand, in some cases it may be
desirable to limit or avoid preheating depending on the materials
in the core glass and the diffusivity thereof into the cladding
material (or vice-versa) at a given temperature or the desired
material concentration profile at the interface of the core and
cladding. Diffusion of material from the core into the cladding may
also be limited to a degree by the speed of the suction process as
well as by the provision of one or more barrier layers within the
cladding tube prior to the core suction process.
[0025] Similarly, the level of vacuum applied is generally not
critical to the practice of the invention but should be great
enough to complete the suction process in a reasonable time (and
with good uniformity if diffusion from the core to the cladding may
be significant) based on the viscosity of the molten core glass but
low enough to avoid any out-gassing from the molten glass other
than above the molten glass column or breakage of portions which
are allowed to solidify during the suction process (e.g. using
reduced cladding tube preheating). The temperature of the molten
core glass will affect viscosity of the core glass which should be
low enough that molten core glass can continuously flow to the
cladding tube bore during the suction process without cavitation or
aspiration of furnace gasses into the cladding tube bore as well as
flowing within the usually very narrow bore of the cladding tube
with an acceptable and practical level of vacuum.
[0026] These and other concerns regarding the interrelationships of
variable process parameters will be apparent to those skilled in
the art in order to predict combinations of process parameters
which might produce flaws in the preforms. However, it is to be
understood that the process is extremely robust, empirical in
nature and may be successfully practiced over a wide range of
values of process parameters and variations. Even the first
experimental trial of the method described above using a mixture of
pure GeO.sub.2 and TeO.sub.2 powders was successful in fabricating
an HNLF optical fiber preform. Therefore, the discussion of
variation in the parameters of the process in accordance with the
invention will principally assist in avoiding combinations of
parameter values which can be projected to be unsuccessful.
[0027] The method has also been successfully and repeatedly
performed with good yield using conventional materials such as
Schott SFL6 glass to form preforms with core diameters ranging from
22 to 45 microns from which optical fibers with suitably small core
diameters, as noted above, have been successfully drawn with good
yield. Other core glass materials successfully used in accordance
with the invention to date also include Lead-Germanate-Telluite
based materials of various compositions and many other glass
formulations with which the invention has provided uniformly
successful results. It should also be appreciated that layers of
other materials such as a barrier layer may be applied within the
cladding tube (e.g. by MCVD) for any desired purpose or
function.
[0028] FIG. 2 shows a cross-section of an optical fiber drawn from
the preform fabricated in accordance with the invention as
described above. FIG. 2 shows the tight confinement of light inside
the core, indicating a very high quality of optical fiber. FIG. 3
shows the multimode output pattern resulting from the launch of
He--Ne light at a 632.8 nm wavelength through the optical fiber of
FIG. 2.
[0029] In view of the foregoing, it is clearly seen that the
invention provides an alternative technique for forming small
diameter preforms from which very small diameter optical fibers of
relatively short length can be drawn, even when using core
materials suitable for HNLF fibers. The method in accordance with
the invention is a rapid, repeatable, robust and economical
technique of producing optical fiber preforms and uses a furnace
which is preferably closed and/or amenable to providing a
controlled atmosphere therein and thus can also confine materials
which may be toxic and controlling sources of potential
contamination which, in any event, are much reduced in number and
likelihood of causing contamination than the greater number of
steps of known processes for making preforms. Potential sources of
other defects such as entrained air are also substantially avoided
in accordance with the invention and toxic materials, if present
may be readily confined.
[0030] The technique in accordance with the invention is
principally suitable for but not limited to, highly doped
non-conventional multi-component glasses. These novel glasses,
mainly used for optical amplification (such as Raman amplifiers and
EDFA amplifiers), lasers, lidar, etc. applications, could either be
tellurium, bismuth, germanium of the like based highly non-linear
glasses or highly doped rare earth glasses like erbium or neodymium
or the like or any other material of combination thereof.
[0031] While the invention has been described in terms of a single
preferred embodiment, those skilled in the art will recognize that
the invention can be practiced with modification within the spirit
and scope of the appended claims.
* * * * *